This invention relates to drilling an underground borehole for installing underground utility lines without excavating a trench. More particularly, the invention relates to a directional rod pusher for moving a string of push rods through the earth to form the borehole. Specifically, the invention relates to an improved directional rod pusher wherein rotational movement of the push rod may be automatically and simultaneously effected upon axial movement of the push rod.
The benefits of trenchless digging for installing underground utility lines are well-known, as disclosed, for example, in U.S. Pat. Nos. 4,306,626 and 4,694,913. A number of different types of devices are available for the purpose of installing underground utility services without cutting an open trench. These include percussion boring tools, rotary boring tools, push rod boring systems, and earth augers. The present invention relates to a push rod boring system. Each of these different types of underground boring devices has a specific purpose and specific operating characteristics. Their use depends on the type of soil in which the borehole will be formed, the length and diameter of the borehole, conditions at the job site, and a number of other factors.
In a typical percussion boring tool, an internal striker or hammer is reciprocated against an anvil or tip to propel the tool through the soil. These tools pierce and compact compressible soils as they form the borehole. A typical percussion boring tool is shown, for example, in U.S. Pat. Nos. 4,621,698, and 4,632,191.
Rotary boring tools use a rotatable "mole" or boring bit to drill through the earth. The mole may be rotated by a downhole motor adjacent the mole or by a surface based drive system. U.S. Pat. Nos. 3,529,682 and 3,589,454 disclose a rotary mole in combination with a complex mole tracking system.
Earth augers are large, powerful screw-type drills for digging horizontal boreholes. These devices are used primarily for digging large diameter boreholes or digging in difficult soil conditions.
A rod pusher is a relatively simple, compact device for sequentially thrusting an increasing string of "push rods" through the ground from a small subsurface starting pit. Such a device can easily be set up and made operational within an hour or two, including excavation of the starting pit. Usually the push rod uses a drill bit having a cutting tip fixed to its leading end. Successive lengths of push rod are pieced together to form a drill string, which forms the borehole. A push rod boring system is disclosed, for example, in U.S. Pat. Nos. 4,306,626 and 4,694,913.
In recent years, new techniques have been developed to allow tracking the progress (i.e., location and depth) of the various types of underground boring devices. Also, there have been various means developed to correct the path of the borehole as the tool progresses, if it begins to deviate from the desired path because of changing soil conditions, rocks, or other obstructions.
In particular, McDonald, U.S. Pat. No. 4,694,913, discloses a rod pusher device having directional control. The directional control is achieved by using a drill bit having an angled or beveled face. As the drill string is pushed through the soil without rotation, the resultant soil forces on the drill bit act at an angle to the centerline of the borehole and string of rods. The perpendicular component of this resultant force tends to cause the head to deviate from its course along a curved path as the string of rods continues to be advanced axially as long as the beveled face of the drill is first maintained in this same orientation, the path of the drill string will follow a continuous curve. An essentially straight borehole can be formed by rotating the beveled drill bit as it is advanced through the soil. When a steering correction is desired, the rotation is stopped with the drill bit oriented to cause deviation of the drill bit back to the desired path. Electronic tracking means known in the art are used to determine the need for path corrections and to indicate the drill bit orientation and thus the orientation of the beveled face.
In order to achieve the rotational motion necessary for directional control of a rod pusher device, McDonald uses a broadly disclosed motor and control assembly which provides either axial movement to the drill string or combined axial and rotational movement. This device requires, however, a complex and expensive control mechanism.
Duke, U.S. Pat. No. 4,306,626 discloses a basic rod pusher device without direction control. It is a very economical directional boring system. This rod pusher device incrementally advances push rods into a bore by gripping the rod with a jaw mechanism that is thrust forward by a hydraulic cylinder. At the end of each cylinder stroke, the jaws are released from the rod and the cylinder is retracted for the next pushing increment. Additional rods are added to the back end of the drill string as needed.
A device such as disclosed in Duke may be made steerable by using a beveled face drill bit attached to the leading end of the string of push rods. However, the simple and economical Duke rod pusher does not have any means for imparting rotary motion to the drill bit. Thus, when a directional boring head is used with this rod pusher, the string of rods must be rotated manually by the crew through use of a pipe wrench or by pushing on the jaw handle. This is a tedious and tiring operation.
The present invention is a simple yet effective means to provide directional control and steering capabilities for rod pusher devices. The invention automatically and simultaneously causes rotational movement of the drill string upon axial movement of the drill string. The invention is used in the context of a simple rod pusher which only has drive means for imparting axial movement to a drill string and does not have any motors or other power sources for causing rotational movement to a drill string.